1. Field of the Invention
This invention relates to an image display method and a device for displaying different aspect ratio images on one screen by sampling input video signals sequentially.
2. Related Art
Since a liquid crystal display device, which is a typical example of an image display apparatus, has such advantageous features as thinness, light weight and low power consumption, it has many applications in personal computers, wordprocessors, television receivers, car navigation devices, projection type display apparatus and the like. An active matrix type liquid crystal display device, above all, has a transistor switching element for each pixel to realize a good quality image without cross-talk between neighboring pixels so that vigorous research and development thereof are still carried out.
The effective display area of a conventional display device has the aspect ratio of 4/3 which is defined by its lateral axis length to its longitudinal axis length. In recent years, however, many display devices have the aspect ratio of 16/9 which effective display are is extended in the horizontal scanning direction and is visually recognized to be large.
There are several known methods of displaying an image with the aspect ratio of 4/3 on the effective display area of a liquid crystal display device with that of 16/9.
According to a first display method, a video signal with the aspect ratio of 4/3, as shown in FIG. 13(a), is sampled sequentially in response to a sampling clock signal to display an image entirely with the aspect ratio of 16/9 as shown in FIG. 13 (b). This method does not require any specific circuit but the original video with the aspect ratio of 4/3 cannot be faithfully reconstructed because the displayed image is deformed in the horizontal scanning direction as schematically shown in 13(b).
A second display method is, as shown in FIG. 13(c) or 13(d), to divide an effective display area with the aspect ratio 16/9 into one with that of 4/9 and another with that of 12/9 (=4/3) where a video signal consisting of an image information with the aspect ratio of 4/3 is displayed.
In this case, however, it is necessary to allocate at least 0.8 H to the display area with the aspect ratio of 12/9 for the sampling period of each horizontal scanning line where H is one NTSC horizontal scanning period and 0.2 H or less to the display area with the aspect ratio of 4/9 for the remaining sampling period. To comply with this requirement, sequential sampling and display are performed based on a video signal which is processed in advance by using frame memories. It does not achieve a less expensive display device of this sort primarily because the frame memories are relatively expensive.
A third display method is described in a Japanese Unexamined Patent Publication (Tokkai Hei) 8-289232. This method is to divide the effective display area with the aspect ratio of 16/9 into two areas, i.e., one with that of 4/9 and another with that of 4/3, and to set sampling clock signals with different frequencies from each other.
In more detail, the frequency of the first sampling clock signal is "3fCK/4" while that of the second "3fCK/2" in the case of the frequency "fCK" of a sampling clock signal used for displaying a video signal with the aspect ratio of 16/9 on the effective display area with that of 16/9.
In order to generate those sampling clock signals, as shown in FIG. 14, a sampling clock generator includes a voltage controlled oscillator VCO, frequency dividers, a phased lock loop PLL with a horizontal synchronous signal input terminal, and a switching circuit SW.
Since the voltage controlled oscillator generates a reference signal with a quite high frequency, it is influenced by outer circuits so easily as to causes various disadvantages in which, for instance, sampling clock signals become unstable, its power consumption increases, and it also generates spurious electromagnetic waves.